154 
S menders . — The Leaf- skin Theory of the Stem 
become merged we get a pattern of eight ridges and five hair lines to the 
internode (see Figs. 12 and 26). 
Lobelia fulgens. Leaf-divergence •§-, leaf-insertion width about of the 
circumference. The contour relations here are the same as those described 
for Veronica, , but the stoutness of the Lobelia stem and its deep red colour, 
against which the colourless hairs are particularly conspicuous, 1 render 
it specially favourable material for 
observation. From the broad leaf- 
insertion there extends down the 
stem on either side a distinct flange 
or wing (see Fig. 30), along the 
margin of which, under certain con- 
ditions, is formed a line of hairs. 
When this is the case, this line of 
hairs is found as high up the stem 
as it is possible to examine the in- 
ternodes, the rest of the surface being- 
completely glabrous. As the stem 
becomes older a further development 
of hairs may take place in response 
to altered conditions, and this may 
result in a general flooding, as it 
were, of previously smooth regions 
with hairs. Under quite other con- 
ditions no hairs at all may be formed, 
and the wing itself may be little 
pronounced. But again we may 
stress the point that the fact that 
the surface may remain free from 
hairs and almost smooth or may 
show a blurring of the original 
pattern is beside the mark. Our 
aim is to prove that certain boun- 
daries exist, and that under suitable 
conditions they become clearly de- 
fined. 
Convolvulus arvensis , Ipomoea coccinea , Ipomoea sanguine a ^ Calystegia 
dahurica (Convolvulaceae). In this family contour ridge lines are a very 
characteristic and conspicuous feature, and are traceable despite any degree 
of torsion in all regions of the stem (see Figs. 19 and 20). In Calystegia 
1 As the anthocyanin in this species occurs in the epidermal layer this distribution is remarkable. 
We have here, as we shall have occasion to note again later elsewhere, a sharp delimitation within 
tissue accompanying different development. At an early stage all the epidermis is coloured, but as 
hairs and guard-cells become differentiated, the colouring matter disappears from these specialized cells. 
Figs. 30-31. 30, Lobelia ftilgens , leaf- 
divergence f ; leaf-insertion approximately f, 
with decurrent wing margined by hairs. 31, 
Cytisus purgans, leaf-divergence -§, leaf-inser- 
tion approximately ^ ; the number of contour 
lines is doubled owing to the additional one 
formed in line with the midrib (nodes numbered 
from below upwards). 
